By means of the calendering of paper in connection with the manufacture of paper, attempts are made to improve the quality values of the paper, or to achieve a higher speed or a better bulk of the paper at an unchanged quality level. The plasticity (workability) of paper can be increased by raising/increasing the temperature and/or the moisture content of the paper. It has been recognized that a considerable change takes place in the plasticity of paper when the temperature of the polymers contained in the paper rises to or beyond the so-called glass transition temperature. At such temperatures, the paper is more readily workable than at temperatures below the glass transition temperature. It is also recognized that an increase in the moisture content of paper lowers the glass transition temperature.
In the art, most commonly the paper web is heated in a calender nip by means of a heatable roll, a so-called "thermo roll", and additionally possibly by means of steam treatment before the nip. The steam treatment is operative to increase the moisture content in the paper and thereby lowers the glass transition temperature. However, it is a problem that at high web running speeds, the paper web do not have enough time to be heated in the nip sufficiently and therefore a portion of the steam escapes into the environment before the calender.
Thus, when paper is calendered, the effect of calendering on the paper is highly dependent on the moisture content and temperature of the fibers contained in the paper at the time of calendering: the workability of the fibers is increased, as if with a drastic jump, when their temperature reaches the so-called glass transition temperature, which depends on the moisture content in the fibers. Above the glass transition temperature, permanent deformations are readily produced in the fibers, and below the glass transition temperature, the deformations tend to be reversible. In order to guarantee the permanence of the effects of calendering, the web can be moistened so as to lower the glass transition temperature and, moreover, it is possible to employ high calendering temperatures and high pressures, in which case the entire web readily exceeds the glass transition temperature and, thus, the deformation takes place evenly across the entire cross section (width) of the web.
From the prior art, it is generally known to heat the paper web before it is passed into the calendering nip. With respect to specific prior art, reference is made to the paper by Dipl.-Ing. Bernhard Krenkel, Glattwerksuntersuchungen, Zusammenhange zwischen Glattparametem und Messgrossen, Dissertation an der Fakultat fur Maschinenwesen und Elektrotechnik der Technischen Hochschule Graz, Heidenheim, Oct. 28, 1975, pp. 40-42, in which preheating of paper before the calendering nip is mentioned so as to improve the calendering result. In this paper, as an example of a possible mode of preheating, a method is mentioned in which the web is made to run along the face of a hot roll before it is passed into the calendering nip.
From German Utility Model Publication No. DE 93 06 448.9, a device is known in which attempts have been made to apply the method described above in practice. In this device, the wrap angle of the web around the hot roll can be regulated. In this prior art method, a problem is the weakness of the contact between the paper web and the roll before the calendering nip, in which case, in reality, the web does not receive a sufficiently rapid preheating.
Another prior art method is described in German Patent Publication No. DE 41 12 537 (which corresponds to U.S. Pat. No. 5,387,782 incorporated by reference herein), in which the web is heated by means of a dielectric electric field before the paper web is passed into the nip. A drawback of this prior art method is the size of the heating device and, because of the size, the significant distance it must be positioned from the nip. In light of this significant distance, the web has time to cool down partially between the heating device and the calendering nip and, moreover, the web has time to deliver (e.g., transfer through evaporation) a considerable part of its moisture to the surrounding air.
Further, with respect to the prior art, reference is made to published Finnish Patent Application No. 923326 (which corresponds to German Patent Publication No. DE 41 26 233 and U.S. Pat. No. 5,318,670 incorporated by reference herein), in which a separate paper web heating device is described. The device is operative before the web is calendered in a nip which is formed by two rolls that are cooled. In this method, the paper web is first heated by means of heat radiators so that the plasticizing temperature is reached on the faces of the paper web, after which the paper web is passed through the pair of rolls that form or define the nip in which the paper web is pressed and cooled. This prior art method has the drawback that the web has time to be partially cooled before the calendering nip and also, is able to deliver a considerable part of its moisture to the surrounding air (e.g., through evaporation).
In published Finnish Patent Application No. 940102 (which corresponds to German Patent Publication No. DE 43 01 023 and Canadian Patent Application No. 2,110,786), a method is described for heating the paper web before it is passed into the calendering nip. In this method, attempts are made to condense steam into the paper web by means of a steam box, which steam delivers the necessary heat to the surface layers of the paper when it is condensed. In this prior art construction, it has been realized that the heating/moistening of the paper web must be carried out as close to the nip as possible in order that the favorable calendering condition produced in the paper is not discharged before the nip. Also in this prior art method, the paper web is, at the same time, moistened and heated before the web is passed into the hot calendering nip.
Reference is also made to published Finnish Patent Application No. 943278 (which corresponds to German Patent No. DE 43 22 876 and Canadian Patent Application No. 2,127,767, in which an arrangement is described having a thermo roll heated by means of an infrared heater.
In general, it is known that in order for the paper web to be worked efficiently, its temperature must be higher than its glass transition temperature, which is about 120.degree. C. to about 150.degree. C. In prior art arrangements, the heat is introduced to the paper web through a hot roll that is in contact with the paper. Other prior art arrangements for heating the paper web include hot-air, steam and infrared heaters. With the use of an infrared device for heating the surface of the paper, it has, however, proven to be a problem that the paper web has become dry before it enters into the nip, and the calendering result has not been improved at all. In the prior art arrangements applying infrared heaters, the reason for drying before the nip has been the long distance between the heating point and the calendering nip.
In a manner in itself known, the paper web can also be coated with a coating which consists of pigment, binder agent, and additives. By means of the coating of the paper web, attempts are made, among other things, to provide the paper with increased whiteness, smoothness and glaze/matt (matted) finish. By means of calendering, all of the above properties are effected. Paper comprises cellulose, hemicellulose and lignin, and coating agents that are used include, among other things, aluminosilicate, calcium carbonate, titanium oxide, and talc, in which binder agents that are used are latexes such as, for example, styrene butadiene and acrylates. The capacity of infrared absorption of all of these agents varies.
Also from the prior art, various elliptic reflectors are known and are used in connection with electrically operated infrared devices. As is well known, such devices are employed in the plastics industry, and by using different coatings of the reflectors, it is possible to produce different reflecting faces. For example, short-wave thermal energy of the sun can be passed in through a glass pane, but the passage of long-wave radiation from the interior outward can be prevented by reflecting it back toward the interior (selective glass). Part of infrared radiation complies with the rules of reflection of visible light.